A problem concerned with speed control of high speed motors, for instance air turbines, is that mechanical speed governors are difficult to get to operate properly at high speed levels due to high dynamic forces, balancing problems etc.
One way of solving this problem is described in U.S. Pat. No. 5,314,299 wherein the rotation speed of a power tool air turbine is governed by a pressure air inlet flow controlling valve which is activated by the air pressure in a pressure sensing opening in the turbine stator. A co-operation between the pressure sensing opening and an oppositely located idle running nozzle in the turbine stator results in a speed responsive activation pressure to be applied on the air inlet flow controlling valve. This previously known device is less advantageous in that it is difficult to obtain an accurate enough control of the motor speed level.